Although various kinds of liquid petroleum resins have been produced hitherto, a liquid petroleum resin which is excellent in color and odor (odorless) and which, when used as a softener for pressure-sensitive adhesives, can impart excellent three tackiness properties has not been obtained. As oils used in the same applications as liquid petroleum resins, various process oils produced from mineral oils are known. However, the uses of these process oils have been limited since in general such process oils unavoidably have the petroleum odor and have been colored. Specifically, for example, in the case of using a process oil as a softener for pressure-sensitive adhesives, the resulting adhesive compositions have been defective in color and odor and, in addition, unable to show sufficient performances with respect to three tackiness properties. Further, although pressuresensitive adhesives containing a highly purified liquid petroleum resin or process oil may be satisfactory in both color and odor, none of such adhesives satisfies the requirement of three tackiness properties.
In general, a pressure-sensitive adhesive is prepared by incorporating a tackifier, softener, and other ingredients into an elastomer as the base polymer. As the softener, process oils of the paraffinic, naphthenic, or similar type produced from mineral oils are most frequently used. Such pressure-sensitive adhesives are being extensively used in the form of tape, label, sheet, or the like for such applications as packaging, sealing, labeling, masking, surface protection, corrosion prevention, electrical insulation, medical uses, and double-side bonding. A pressure-sensitive adhesive tape, for example, is produced by the following method.
A diene-type polymer, such as natural rubber, styrene-butadiene rubber, or isoprene rubber, as an elastomer is dissolved in an organic solvent along with a tackifier and softener and with, according to need, a stabilizer, filler, and other additives thereby to prepare a solution-form mixture of these ingredients. This solution is then coated on a tape material, release paper, or other substrate and the coating is dried to evaporate and remove the solvent, to produce a pressure-sensitive adhesive tape.
However, from the standpoints of safety, environmental pollution prevention, and production efficiency, production of pressure-sensitive adhesive tapes by a hot melt technique using no organic solvent is spreading rapidly in recent years. In this hot melt production technique, a mixture of a base polymer, tackifier, softener, and other ingredients is melted by heating and the melt is applied on a tape material or the like. The adhesive mixture is therefore required to have an adequate melt viscosity so as to attain good coating properties and production efficiency.
As the base polymer for such a hot melt pressuresensitive adhesive, various polymers are employed such as block copolymers of a conjugated diene and a vinyl-containing aromatic hydrocarbon (hereinafter referred to simply as block copolymers), ethylene-vinyl acetate copolymers, ethylene-.alpha.-olefin copolymers, polyester resins, and the like. Of these, the block copolymers are being used in increasing quantities because they have a relatively well balanced combination of tackiness and creep resistance.
As such block copolymers, various ones have been commercialized, including styrene-isoprene block copolymers, styrene-butadiene block copolymers, and hydrogenated styrenebutadiene block copolymers.
As the tackifier, natural resins such as rosins and terpene resins and petroleum resins such as aliphatic or aromatic resins and these aliphatic and aromatic hybrids are frequently used. As the softener, liquid resins are generally incorporated, such as process oils obtained from mineral oils, plasticizers, polybutene, and liquid rubbers.
The pressure-sensitive adhesive having such a composition is required to have an adequate viscosity in order that even when being pressed lightly, e.g., pressed by a finger, it can come into sufficient contact with the adherend surface to have a sufficiently large contact area in terms of microscopic contact area. Furthermore, the pressure-sensitive adhesive should also have elasticity so as to withstand external forces including peeling and shearing forces.
From the standpoint of smoothly conducting coating operations in the production of a pressure-sensitive adhesive product by the hot melt technique, there is an adequate viscosity range for the hot melt pressure-sensitive adhesive. In general, hot melt pressure-sensitive adhesives are required to have lower melt viscosities than solvent-based and other pressure-sensitive adhesives.
Although process oils are used to lower melt viscosity and this results in an improvement in tack, incorporation of process oils produces adverse effects, e.g., a decrease in holding power. Accordingly, there is no process oil which, when incorporated in a hot melt pressure-sensitive adhesive, enables the adhesive to have a well balanced combination of tack, adhesion strength, and holding power which are called three tackiness properties.
The conventional hot melt pressure-sensitive adhesives are also insufficient not only in adhesion to nonpolar polyolefin adherends especially at low temperatures, such performance being recently desired increasing in the industrial fields, but also in holding power.